Field of the Invention
The present invention concerns: a method to generate a magnetic resonance (MR) phase contrast angiography image of an examination subject, in which velocity-dependent phase information is impressed on moving spins in the examination subject by activating additional bipolar coding gradients; and an MR system for implementing such a method.
Description of the Prior Art
Magnetic resonance angiography generates MR images of the vascular system of an examination subject. Two basic angiography techniques are known. The first technique is based on the phenomenon of Time of Flight (TOF) effects, in which differences in the signal saturation that exist between flowing blood and stationary tissue are used. The other technique of MR angiography is based on the fact that phase information is impressed on moving spins, which phase information differs from the phase information of non-moving spins. For this purpose, two MR images are normally acquired in phase contrast angiography technique, namely an MR image without additional (for the most part bipolar) coding gradients and an MR image with switching of the additional bipolar coding gradients. By calculating the phase difference of the two images, or by a complex difference calculation of the two images, velocity information results from the phase difference along the direction along which the additional bipolar coding gradient was switched (activated).
An image sequence given which a flow information in three different spatial directions can be generated by means of phase contrast angiography is shown in a schematically simplified manner in FIG. 1. An RF pulse 10 is activated during a slice-selection gradient 11 to excite spins in a slice. As in other imaging sequences, a phase coding gradient 12 and a readout gradient 13 are switched, wherein the signal readout takes place during a time period 14 during the readout gradients. The time period from the activation of RF pulse 10 the generated signal echo is called the echo time TE. Additional bipolar coding gradients 15, 16 and 17 can now be switched in order to respectively obtain a flow information along the spatial directions X, Y and Z. As is apparent from FIG. 1, four measurements are typically required for a three-dimensional flow information: one reference measurement without switching of additional bipolar coding gradients and a respective measurement with switching of an additional coding gradient in one of the three spatial directions. The gradient moment generated by the bipolar coding gradients is established by the maximum velocity to be coded.
Compared to the base sequence without additional coding gradients, this initially leads to an increase of the minimal echo time since the readout gradient 13 cannot be switched directly after the phase coding gradient 12; rather, a time period must additionally be provided for switching of the bipolar coding gradients. These bipolar coding gradients (also called Venc gradients) can be temporally superimposed on the gradients used for the underlying imaging sequence in order to minimize the echo time. In addition to shortening the measurement duration, thus leads to additional advantages with regard to the quality since (for example) the unwanted effect of an intravoxel dephasing (i.e. the destructive superposition of different velocity components) is reduced, whereby a signal loss due to the T*2 [sic?] decay is correspondingly reduced.
The superposition of the bipolar coding gradients with the imaging gradients can be achieved with Cartesian k-space sampling, as shown in FIG. 1. As noted, four data sets are typically created: one with flow-compensated gradient scheme without additional bipolar gradient moment, and three additional data sets with respective bipolar coding gradients spatially orthogonal to one another.
FIG. 2 shows how the flow information is acquired in the x-direction and y-direction in a two-dimensional case, wherein the readout direction always runs in the x-direction. As is apparent to the left in FIG. 2, given Cartesian coordinates the readout direction in the shown example always takes place in the x-direction. The additional bipolar phase coding gradient 17 is switched once in the x-direction, whereby a flow coding takes place in this direction, and once in the y-direction, whereby a flow coding takes place in the y-direction. A third measurement takes place without additional flow coding gradients.
It is additionally known to acquire the raw data space or, respectively, k-space belonging to an MR image with non-Cartesian k-space trajectories. In particular, a greater time efficiency is achieved via an undersampling.
The three-dimensional radial k-space sampling can also be combined with the velocity coding described above in order to achieve an efficient 3D measurement with vector flow coding as it is described in U.S. Pat. No. 6,188,922.
Such an acquisition scheme is schematically shown in FIG. 3. The coding gradients 15-17 along the three Cartesian spatial directions are maintained; however, the gradients 18, 19, 20 of the data readout are switched so that the desired (for example radial) k-space sampling is achieved. The variations of the individual gradients that are respectively shown for the gradients 18, 19, 20 should show the variation of the individual gradients for each radial k-space projection. In FIG. 4, this is shown for the two-dimensional case. For a radial readout with a trajectory 8 (in the shown case from the lower left to the upper right), the gradient switching takes place in the x-direction and y-direction of gradients 19 and 20 such that the projection 8 is achieved. As in FIGS. 2 and 3, however, the additional bipolar coding gradients furthermore take place in the fixed spatial directions x and y via switching of the additional bipolar gradients 16 and 17. As is apparent in FIGS. 3 and 4, the additional bipolar coding gradients are switched in a spatially fixed, physical xyz-coordinate system, in contrast to which the imaging sequence given a three-dimensional radial sampling includes a readout gradient rotating in a solid angle with every readout. For each projection, a gradient curve that is different in both reference systems would thus result from the temporal superposition of the spatially fixed flow coding gradients with the rotating readout gradients. This means that, in the entirety of the gradient trains that are used for a complete k-space sampling, only a single one can experience an optimization according to the minimization of TE; all other gradient curves cannot be realized in a time-optimized manner. The echo time TE is, however constant as a global measurement parameter for all measured k-space projections. This inevitably leads to an extension of the TE time in comparison to the time-optimized superposition.
This is explained in connection with FIGS. 5 through 8. In FIG. 5, the additional bipolar coding gradient is represented with 17 in dashed lines. Given a radial readout, a k-space trajectory results once that likewise takes place along the coordinate axis of Gread. This readout gradient is represented with 20 in FIG. 5. If the bipolar coding gradient 17 and the readout gradient now have the same polarity and a temporal minimization is attempted by superimposing the two gradients (as is symbolized by the arrow in FIG. 5), the image at FIG. 6 results, where the bipolar coding gradient 16 and the readout gradient 20 are superimposed. This means that—as shown in FIG. 7—a gradient switching would be necessary, as is represented by the gradients 21A and 21B. The gradients 21A and 21B correspond to the constructive superposition of the bipolar coding gradient 17 with the flow-compensated pre-gradients of the readout gradient 20. As is in particular apparent from the gradient curves 21A and 21B, to shorten an echo time a gradient strength that is very high overall is necessary, as well as a fast slew rate of the gradient. If this is not possible due to the existing gradient system, or if such a gradient switching is not desired for other reasons, as shown in FIG. 8 this can only be replaced by an extended switching of the gradient in order to achieve the same gradient moment as in FIG. 5 via the gradients 21A and 21B. This means that overall the echo time TE is extended since the gradient 22 must be switched over a longer time period in order to generate an identical gradient moment as the gradients 21A and 21B of FIG. 7. However, this leads to an extended echo time. This extended echo time occurring in a projection must then be used in all other projections since the echo time must be kept constant for all projections.
This means that, in the prior art, a lengthening of the echo time overall has been accepted.